There are two main validation tools. One it EpubCheck and the other is Flightcrew. Flightcrew is also built-in into Sigil.
In general Flightcrew/Sigil will give you better understandable error messages.

Be aware though. They will check whether your ePUB is structurally correct. That does not always mean that your ePUB will render as you would like. That depends on the reader application.

Another tools which is quite valuable, is the CSS validation tool. An error in the stylesheet will often result in the reader application completely ignoring the stylesheet.

If I'm understanding the problem correctly, I think it's less a problem with ADE and more a issue with InDesign. Sounds like your formating the book in InDesign, exporting to ePub, then opening it in ADE, and the formatting you are seeing in ADE are not what you created in InDesign, correct? If so the problem is InDesign's ePub export cannot convert everything that you can do in InDesign to ePub. InDesign is great for creating fixed format publications (print/PDF), but will never be able to convert everything to ePub. The usual workflow with InDesign is to create your document, export to ePub, then use an ePub editor like Sigil to fix all the things that InDesign cannot convert correctly.
I don't know if there are any tutorials here on MobileRead or for free on the 'net on the process of doing this, but there are some good ones at lynda.com (unfortunatly they are not free).

Yes, I understand that I can't go crazy in InDesign and hope to have it show up in an ePub, so I am being very conservative in the layout.

My question was really what application I should be checking it with before sending it off to BookBaby, not from a validation standpoint but from a formatting perspective. I found that ADE displayed some strange spacing between some paragraphs, while it looked fine in Sigil. And an Adobe tech went on record and said use "anything but" ADE to proof an ePub. Sounds like there is no definitive proofing tool.

Thanks for the CSS validation tool link, I wasn't aware of that. I can confirm that Lynda.com does indeed have one useful tutorial series devoted to creating ePubs with InDesign. Elizabeth Castro's From InDesign CS 5.5 to EPUB and Kindle is also very good.

... I found that ADE displayed some strange spacing between some paragraphs, while it looked fine in Sigil. And an Adobe tech went on record and said use "anything but" ADE to proof an ePub. Sounds like there is no definitive proofing tool...

Correct, there is absolutely no definitive proofing tool for ePub.

The reason ADE is frequently recommended around here as one component in proofing is that it is the 800# Gorilla in the reader market. It is the software engine in most ereaders. If a book doesn't look good in ADE it won't look good on most ereaders in use today.

Cool, it wasn't clear if you were tweaking in Sigil after creating in InDesign.

Quote:

"anything but" ADE...

Problem with that is if you're creating an ePub for anything other than iBooks or conversion to Kindle, it's most likely that the reader is going to be using an ADE engine, so ADE is what you would use for proofing.

With regards to the odd spacing, without seeing the underlying code I would guess there is something in the CSS that is not supported in ePub/ADE but works in Sigil's book view (Sigil's rendering engine supports things that are not in the ePub spec). You'll have to take a look at the code to see if you can find what's causing it.

What I don't get is why people feel that using programs such as Sigil, The FF extension, Calibre's viewer are OK to use in place of ADE to check out their ePub. ADE is used by most readers and if they use ADE, then they will know better how it may look on a stand alone reader.

What I don't get is why people feel that using programs such as Sigil, The FF extension, Calibre's viewer are OK to use in place of ADE to check out their ePub. ADE is used by most readers and if they use ADE, then they will know better how it may look on a stand alone reader.

Lack of knowledge, which is something that everyone has been guilty of at some point. Which I think is the whole point of having this forum, to help spread the knowledge that we all took the time to learn (sometimes the hard way) to those who aren't quite as far along as we are (I know I still have got a lot to learn). Plus many people expect their documents to look the same no matter what it's being viewed on, especially in publishing where WYSIWYG is almost a given. Unfortunately one of the problems I have with ePub is that so much the spec is optional (or outright ignored or broken) in the rendering engines, that it's impossible to know what something will look like in any specific reader just by looking at it in one.

What I don't get is why people feel that using programs such as Sigil, The FF extension, Calibre's viewer are OK to use in place of ADE to check out their ePub. ADE is used by most readers and if they use ADE, then they will know better how it may look on a stand alone reader.

I only use those as my first pass check.
If it looks wrong there, there is Zero chance it will look/behave better in ADE

Keep in mind RMSDK version when thinking about previewing. ADE 1.7 uses a much older version of RMSDK - this will be equivalent to what the older Sony Readers (or Nook Classic) is using. ADE 1.8 (in labs.adobe.com) is using the current version of RMSDK. One of the big changes between the two versions is that a new text rendering engine was added, and the layout engine was tweaked accordingly (hyphenation was added for instance). For most content you will get a noticeable difference in results (and for some content there will be a dramatic difference).